Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory Parkinson's Disease, and DBS has also recently been applied in additional areas, such as essential tremor and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (i.e., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. For example, in the context of SCS, the electrode lead(s) are typically implanted along the dura of the spinal cord, with the electrode lead(s) exiting the spinal column, where they can generally be coupled to one or more electrode lead extensions. The electrode lead extension(s), in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket (typically in the chest or abdomen) where the neurostimulator is implanted.
The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer, which may take the form of a remote control (RC) may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue. In particular, electrical energy conveyed between at least one cathodic electrode and at least one anodic electrodes creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neurons beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers.
Electrical pulses may be delivered from the neurostimulator to the stimulator electrode(s) in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, and rate of the stimulation pulses.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing current pulses (anodes) and which of the electrodes are sinking current pulses (cathodes) at any given time, as well as the magnitude, duration, and rate of the electrical current pulses. The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. In the case of SCS, such a therapeutic benefit is “paresthesia,” i.e., a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
Electrical energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar delivery occurs when a selected one or more of the lead electrodes is activated along with the case of the neurostimulator, so that electrical energy is transmitted between the selected electrode and the case. Bipolar delivery occurs when two of the lead electrodes are activated as anode and cathode, so that electrical energy is transmitted between the selected electrodes. Tripolar delivery occurs when three of the lead electrodes are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode.
In the context of SCS, the neurostimulator case is used as a cathodic return electrode, and the lead electrodes are used as anodic stimulating electrodes. The neurostimulator case is selected as the cathodic return electrode, because it is relatively far away from the stimulation site, and because it has a large surface area, resulting in relatively small current densities.
This pocket stimulation problem is exacerbated when microstimulators are used. A “microstimulator” is an implantable neurostimulator in which the body or case of the device is compact (typically on the order of a few millimeters is diameter by several millimeters to a few centimeters in length). For example, the Bion® microstimulator (manufactured and distributed by Boston Scientific Neuromodulation Corporation) is a tiny fraction of the size of the Precision® IPG. Typically, the cases of the microstimulators carry electrodes for producing the desired electrical stimulation current. Microstimulators of this type (i.e., microstimulators with leadless electrodes) are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. In these cases, it is, of course, desired for the pocket in which the microstimulator is implanted to be stimulated.
However, it may sometimes be desirable to connect one or more short, flexible stimulation leads to a microstimulator, as described in U.S. patent application Ser. No. 09/624,120, filed Jul. 24, 2000, which is expressly incorporated herein by reference. The use of such leads may permit electrical stimulation to be directed more locally to target tissue a short distance from the microstimulator, while allowing the microstimulator to be located in a more surgically convenient site. In this case, stimulation of the implantation pocket is undesirable.
Because the case of a microstimulator is relatively small, the current density on the surface of the case may be quite high when the microstimulator is operated in a monopolar mode. For example, the surface area on the case of a Precision® IPG is 3882 mm2, whereas the surface area of the anodic surface of the Bion® microstimulator is approximately 50 mm2. If this anodic surface were used with a leaded Bion® microstimulator, undesired and perhaps annoying or painful stimulation in the implantation pocket might be expected.
Attempts have been made to prevent or, at least reduce, inadvertent pocket stimulation when operating a neurostimulator in a monopolar mode. For example, it is known to coat a portion of the neurostimulator case (e.g., the edges where current density is the greatest) with an insulative material in order to reduce pocket stimulation (see Toshimi Yajima, et al. “Effects of Muscle Potential Depression and Muscle Stimulation Caused by Different Insulation Coating Configurations on Cardiac Pacemakers: The Use of Insulative Coatings to Try to Reduce Pocket Stimulation,” J Artif Organs (2005) 8:47-50; Davies T, “Do Permanent Pacemakers Need an Insulative Coating? Results of Prospective Randomized Double-Blind Study,” Pacing Clin. Electrophysiol. 1997 October; 20(10 Pt 1):2394-7). However, coating a portion of the neurostimulator case necessarily increases the current density of the uncoated portions, thereby potentially increasing the chance that pocket stimulation will occur adjacent these higher current density sections. Furthermore, new edges are created between the coated and uncoated portions of the neurostimulator case, thereby creating higher current densities at these new edges.
There, thus, remains a need to provide an improved neurostimulator and technique that prevents, or at least, minimizes inadvertent pocket stimulation.